no. 1630. ARM HOMOLOGIES IN RECENT CRINOIDS— CLARK. 121 



are all of two joints, joined by synarthry (or, more rarely, syzygy), 

 the distal faces of the axillary are always straight muscular articula- 

 tions. Thus we see that, whereas in the Antedoimhe and ten-armed 

 genera and species of other families (except the Pentametrocrinidse 

 and Uintacrmidae) the first post-radial joint and the axillary are 

 merely repetitions of Z 1 and Z 2 interpolated between Z x and the ra- 

 dials, so we find that all the division series, no matter how many 

 there are, are all additional repetitions of Z t and 

 Z,. interposed between the true Z, and Z 2 and tO^^^JC^ 



the first post-radial reduplication of those joints. NIFFw 



When the division series consist of four instead \^~/ 



of two joints, it is merely a case of a doubling of ^— ' 



the more common primary two, so that, instead fig.22.— comasteridjh; 



- . .... . - . . ,. ,. . . COMATULA. 



ot single division series 01 two joints the division 



series are double, the two component pairs being united by a straight 

 muscular articulation like that on the distal face or faces of the second 

 joint of a division series of two joints only. It is interesting to note 

 that in Thalassometra gigantea, in which species Z 2 bears a very sharp 

 median keel, quite lacking on all the other joints of the free undi- 

 vided arm, this keel is repeated on the second post-radial joint (first 

 axillary), the second and fourth (the latter an axillary) joints of 



Fig. 2.'>. — Comasterid.e ; Comatola. 



division series of four joints (the remaining first and third joints 

 being quite without it), and the second joint of division series con- 

 sisting of two joints. 



In the young ten-armed stage of all comatulids, so far as I have 

 been able to find out, Z x is always the first post-axillary (third post- 



